Most frozen products have poor thermal conductivity since ice is a poor conductor of heat. The poor thermal conductivity significantly slows down the freezing process, which for many frozen products negatively affects their quality. If the frozen product is additionally foamed, e.g. in ice creams, the thermal conductivity is especially poor, and therefore the quality issues due to slow freezing are more pronounced.
Commercially produced ice cream products are essentially an emulsion which is foamed and frozen. FIG. 1 illustrates and outlines a standard process flow diagram for the current method of manufacturing frozen food foams such as ice creams. The ingredients (105) are first mixed together (110) and then pasteurized (115) to kill micro-organisms. The mix is then homogenized (120) by passing through high pressure valves in order to provide a more stable oil-in-water (o/w) emulsion. The homogenized mix is then mixed with the desired flavors and colors (125) and allowed to age, for instance at approximately 40 degrees Fahrenheit for a suitable time such as 4 to 8 hours. During the ageing process (130), multiple changes occur on the surface of the dispersed fat droplets contained within the homogenized mix and this prepares the mix for partial coalescence. Next, the mix is partially frozen, typically in a scraped surface heat exchanger or freezer. Air (135) is normally introduced into the mix during this freezing process (140) or in some cases the air can be provided just prior to the freezer using various kinds of pre-aerators. Next, any inclusions and/or variegates (145) are added to the product. The product is then packaged (150) and finally hardened. Hardening (155) is a more complete freezing of the product to reduce the unfrozen fraction of the product while also increasing viscosity and is typically carried out in mechanical blast freezers. For some products, like ice cream novelties, the hardening step may occur before the product is packaged.
The ice cream manufacturing steps generally include: mixing the liquid and dry ingredients; pasteurization and homogenization (in either order) followed by addition of flavors and colors. The product is then aged during the ageing step, after which, air is introduced. During the ageing step, the homogenized and pasteurized mixture is generally cooled down to about 35 degrees Fahrenheit to 40 degrees Fahrenheit and stored for at least 4 hours in tanks with minimum agitation. The main purpose of the ageing step is to promote fat destabilization. In other words, the ageing step provides time for the emulsifiers in the product to displace proteins from the fat globule surface which reduces the membrane thickness of the fat globule and makes it more susceptible to coalescence. Also, the ageing step provides time for partial fat crystallization, so that the fat globules can also partially coalesce. In general about 30 percent to about 50 percent of the fat crystallizes during the typical ageing step and the fat droplets partially coalesce yielding a fat droplet network.
After ageing, the foam product is next directed to a conventional scraped surface heat exchanger where the product is subjected to the freezing and aerating step. The air may be incorporated into the aged mix prior to the scraped surface heat exchanger, using a device such as a mechanical pre-aerator, or may be directly introduced into the scraped surface heat exchanger. The typical residency time of product within the scraped surface heat exchanger is normally between about 30 to 120 seconds. During this freezing and aerating step, the air bubbles are incorporated, broken into smaller bubbles, and distributed within the ice cream product while the product undergoes partial freezing (e.g. about 30 to 50 percent of the water in the ice cream is frozen). The ice cream product is also normally whipped or agitated to further promote fat destabilization. Upon exiting the scraped surface heat exchanger, the ice cream product exhibits a temperature of about 20 degrees Fahrenheit and a viscosity typically between about 1000 to 5000 centipoise.
Upon exiting the scraped surface heat exchanger, the foam product is subsequently packaged and then hardened in a spiral or tunnel freezer. The targeted final temperature of the ice cream product is between 0 degrees Fahrenheit and minus 20 degrees Fahrenheit resulting in freezing about 60 to 85 percent of the water in the ice cream product. The residence time in the hardening freezer very much depends on numerous parameters including the size of the ice cream packaging and “overrun”. The term “overrun”, is used to indicate how much air or other gas a particular ice cream contains. It is basically the ratio of the volume of the ice cream, less the volume of the liquid ice cream mix, divided by the volume of the liquid ice cream mix. So, if 50 percent of the volume of the ice cream is air, the overrun would be 100 percent.
In frozen foams such as ice creams, increasing the overrun results in a decrease in the percentage of other ingredients (e.g. milk fat, carbohydrates, stabilizers, etc) required, which in turn results in cost savings. Of particular value in ice cream products is reduction of the milk fat ingredient which allows for improving the dietary and nutritional characteristics of the ice cream product. Typically one would limit the overrun due to regulatory based restrictions on overrun (e.g. maximum allowable overrun) or because the overrun adversely affects the sensory and physical properties of the foam product that may occur with too much of an increase in overrun.
During the hardening step, the mechanical freezers chill the outside of the ice cream surface or packaging material with cold air, and rely on the thermal conductivity of the packaging materials and the ice cream product to chill the rest of the ice cream. However, foams like ice cream are very poor thermal conductors. Due to this, the center of the ice cream takes a long time to chill during the hardening process. Thus, during a large part of the hardening process, the viscosity of most of the ice cream is low. Thus, typically, ice crystals and air bubbles in the ice cream product rise sharply in size during the packaging and hardening steps. The quantity and size of these bubbles greatly influence the physico-chemical properties of the final foam product. In particular, there is a significant increase in average bubble size and coalescence during the hardening step due to gas bubble disproportionation and coalescence.
The ingredients typically used in most commercially available ice cream products consists of: (i) milkfat; (ii) milk solids not fat (MSNF) such as proteins, casein, whey proteins, etc.; (iii) carbohydrates (e.g. lactose) and sweeteners, such as sucrose or corn syrup; (iv) water; (v) stabilizers and surfactants, including gelatins, gums, sodium alginate, carrageenan, etc. and surfactants; (vi) emulsifiers, such as mono-glycerides, di-glycerides, polysorbates, polyglycerins, and combinations thereof; and (vii) air or other gas bubbles.
In general, the milk fat typically represents about 10 to 16 weight percent of the liquid ice cream mix and provides flavor, texture and smoothness to the ice cream. A continuing challenge for ice cream manufacturers is to lower the milkfat content in the ice cream product while maintaining the sensory feel and taste of the ice cream.
The MSNF, and more particularly, the proteins within the MSNF, improve the texture of the ice cream (e.g. body and bite) and also help emulsify and whip the fats during manufacturing of the ice cream product. The carbohydrates, sweeteners, and any added flavorings are included to generally improve the taste of the ice cream, including sweetness, palatability, and texture. The carbohydrates also tend to aid in freezing point depression of the ice cream product which improves the scoopability of the ice cream product. The water represents about 55 to 64 weight percent of the liquid ice cream mix and provides the source of ice crystals in the ice cream product. If the ice crystal content is properly controlled this tends to also improve scoopability. The stabilizers and surfactants are used to add stability to the ice cream product during and after manufacture and possibly improve the sensory feel of the ice cream upon consumption. Finally, the emulsifiers are used primarily for fat destabilization through displacement of proteins on the surface of the fat droplets.
Typically, about 30 to 50 percent of the total ice cream volume is either air or another gas which functions to improve the taste (e.g. creaminess) and texture desired by customers.
Stability of the final ice cream product is achieved by controlling the size and distribution of fat globules, ice crystals, and air bubble globules in the ice cream product. Optimized fat globule size and distribution is often achieved during the homogenization, ageing, and freezing steps in ice cream manufacture. During the ageing and freezing of the ice cream mix, the fat droplets partially coalesce to form a structural network within the liquid ice cream mix and this network of fat droplets coats the surface of the introduced air bubbles to provide stability (See FIG. 2). FIG. 2 depicts an enlarged view illustrating the key constituents of the micro-structure of a typical food foam product (during processing). The gas bubbles (210) and ice crystals (220) are generally well dispersed throughout the continuous phase of the unfrozen liquid phase (240). During the manufacturing process the fat droplets undergo partial coalescence yielding a 3-dimensional network that provides a support structure for the product. This network of partially coalesced fat droplets (230) tends to stick to the surface of the gas bubbles due to their hydrophobic nature and therefore contribute to the stability of the bubbles. The lower the level of fat, the lower the resultant gas bubble stability of the product. During the initial stages of hardening, the microstructure is most susceptible to change. This is mainly due to the low viscosity of the continuous phase which allows for bubble and ice crystal growth and channeling. Once hardened, the rate of change is extremely slow.
Ideally, an ice cream manufacturer would seek to develop an ice cream product with the smallest size and most uniform distribution of air or gas bubbles and ice crystals and retain this uniform dispersion of air or gas bubbles and ice crystals both during and after manufacture.
One manifestation of the stability problem in many foam products such as ice cream (as well as whipped cream, icing or topping) is commonly referred to as the “altitude problem”. The altitude problem is defined as the degradation in the quality and stability of foam products during transportation or storage, due to pressure variations resulting from altitude changes occurring en-route. For example, when foam products are transported from a low altitude location to a high altitude location the ambient pressure proximate the foam product decreases. This change in ambient pressure causes the gas in the foam product to expand, which in turn adversely impacts the stability of the foam structure. In many cases, this gas expansion in the foam product results in coalescence of the individual gas bubbles and ultimately leads to a channeling effect and escape of the gas from the foam product. Because the trapped air or gas bubbles form a significant portion of the total foam product volume, any change in volume of trapped air or gas bubbles due to pressure variations may lead to product damage, leakage and, in some cases, container deformation during shipping of the foam product to higher altitudes. On the other hand, when the expanded foam products are transported from the high altitude location to a lower altitude location, the ambient pressure proximate the foam product increases causing contraction of the foam, which causes the product to appear to have shrunk.
Another manifestation of the stability problem is “shrinkage”. Shrinkage itself is an umbrella term that describes the reduction in volume of the final product such that the package appears only partially full. The main reason for shrinkage is a lack of bubble stability that causes coalescence and further escape of the gas from the product. Changes in altitude can precipitate shrinkage as described above, but in many cases product stored at constant pressures will also undergo shrinkage.
There is therefore, a continuing need in the industry for a method to improve the stability, homogeneity, and quality, and to reduce the manufacturing cost without adversely impacting the quality of ice creams and other food foams. In particular, there is a need to reduce or mitigate the stability problems associated with altitude in many ice creams and other food foams such as whipped products, icings and toppings as well as refrigerated, partially or fully frozen forms of the same.